Some embodiments of the inventive subject matter relate to circuits for devices such as contactless cards and, more particularly, to clamp circuits for devices such as contactless cards.
Contactless card systems, particularly smart card systems, use contactless information recognition technology. For example, a card reader recognizes a smart card in a distance of several centimeters using a radio frequency and transmits and receives information to and from the recognized smart card.
A contactless card typically has a unique identification (ID) number. A contactless card typically includes a central processing unit (CPU), a data memory, and an interface and performs contactless short-range communication with a card reader connected to a host computer.
Some conventional contactless cards generates an alternating current (AC) voltage from electromagnetic waves generated from the card reader based on electromagnetic induction of an antenna. A rectifier circuit included in the contactless card may convert the AC voltage to a direct current (DC) voltage. The DC voltage may be regulated by a regulator circuit. The stable voltage produced by the regulator circuit is provided as power for a logic circuit.
The intensity of electromagnetic waves between the card reader and the contactless card may vary with environment. In particular, when the electromagnetic waves generated from the card reader increase quickly in a short time, e.g., 1 μs, overshoot of an internal voltage of the contactless card may occur because a bandgap reference voltage and the regulator circuit in the contactless card have not settled.